1. Field of the Invention
The present invention relates to a multiplexer, a transmission apparatus, and a reception apparatus.
2. Description of the Related Art
In recent years, cellular phones have required that a single terminal support multiple frequency bands and multiple wireless systems, that is, multiband and multimode terminals have been required. To this end, a multiplexer for separating high-frequency signals according to a radio carrier frequency (band) is arranged immediately below an antenna. As a plurality of band pass filters of a multiplexer, elastic wave filters exhibiting low-loss characteristics in a pass band and steepness near the pass band are used.
Japanese Unexamined Patent Application Publication No. 2003-332885 discloses a surface acoustic wave (SAW) demultiplexer in which two ladder SAW filters having different pass bands are connected to a common terminal. In the SAW demultiplexer, an impedance matching circuit including an inductor and a capacitor is arranged between an antenna and the common terminal.
Furthermore, Japanese Unexamined Patent Application Publication No. 2004-104799 discloses a filter in which an acoustic resonator is arranged between a connection node of inductors connected in series and ground. With this arrangement, a notch filter with a high steepness is formed by an LC resonant circuit including the inductor and the capacitance of the acoustic resonator.
For a multiplexer having a small number of bands to be used, impedance matching between an antenna element and each signal path may be achieved by arranging an impedance matching circuit between the antenna element and a common terminal, as with the SAW demultiplexer described in Japanese Unexamined Patent Application Publication No. 2003-332885.
However, as the number of bands to be used increases, it becomes more difficult to achieve impedance matching for a plurality of filter elements using only the above-mentioned impedance matching circuit. Thus, adding, in addition to the above-mentioned impedance matching circuit, matching elements such as an inductor and a capacitor to individual filter elements, has been considered. In this case, however, a notch waveform formed by the LC resonant circuit described in Japanese Unexamined Patent Application Publication No. 2004-104799 may be unexpectedly generated within a pass band of the multiplexer. As a result, it becomes difficult to ensure low-loss characteristics in the pass band of the multiplexer.